Class MutableArray

This mixin defines the API for modifying array-like objects. These methods
can be applied only to a collection that keeps its items in an ordered set.
It builds upon the Array mixin and adds methods to modify the array.
One concrete implementations of this class include ArrayProxy.

It is important to use the methods in this class to modify arrays so that
changes are observable. This allows the binding system in Ember to function
correctly.

Note that an Array can change even if it does not implement this mixin.
For example, one might implement a SparseArray that cannot be directly
modified, but if its underlying enumerable changes, it will change also.